TASK: Ensure initialized account before creating client secret lookup

[sorenmalling]

In a combination with a custom ThumbnailGenerator from the Sitegeist.AssetSource.3QVideo package (https://github.com/sitegeist/Sitegeist.AssetSource.3QVideo/blob/main/Classes/Domain/ThumbnailGenerator.php) there is no Account in the security context, when going through the ThumbnailStrategies on a media/thumbnail/<uuid> url.

This change adds a check for a actual account, before trying to fetch a client secret

Resolves #16

[sorenmalling]

ping @mficzel

[sorenmalling]

Disclaimer: There is a posibility of this being a bug in the Media module. The tokens are "gone" in between the initialize and isInitialized call, when this Pixx.io, does the check, where I added the extra condition.
Me and @mficzel are curious to find out how/what/where/why this happens. We both believe that there can be a different solution that this, that is more focused on a core issue.

[kitsunet]

Makes sense to prevent errors here even if in a "default" use case it should have an account.

[sorenmalling]

@kitsunet Will you take care of merging and create a release together with #14 ? :)